OVERVIEW

The 17-story Factor building was equipped with 72 seismometers after the 1994 Northridge earthquake. In the first year of CENS, we converted 24 channels of the on-site recording system to continuous network recording. Last year we ordered equipment to convert all 72 channels to real-time recording viewable via the Internet. Factor is now the ‘most heavily instrumented’ building in North America and serves as a CENS testbed for building state-of-health monitoring. To extend the Factor building testbed area we propose to install a borehole seismometer 50-100 m deep and an array of 5 seismometers in a N-S E-W cross centered on Factor with 100 m spacing.

APPROACHes

The first objective is to use Internet technology for real-time monitoring and to develop algorithms to detect changes in the building’s state of health especially during and after an earthquake. The second objective is to completely characterize the incoming earthquake waves to the building at depth in the bedrock before they reach the soil where non-linear effects such as strong attenuation and dramatic reduction of shear modulus occur. Also we have found that the amplitude of incoming waves can depend critically on direction because of focusing and defocusing effects by the geology. As a result, we need a ground array to characterize the incident directionality.

We toured the UCLA tunnels and have negotiated placing sensors in the tunnels to make up a permanent ground array. We have an agreement with the USGS ANSS program to obtain instrumentation for 5 ground stations 100 m to the N, S, E, and W of the building. We envisage using the existing optical cables installed by Facilities for the network connectivity. We will need to radio-feed time into the tunnels. The portable network will use wireless connections and the CENS timing protocols under development.

We were awarded a USGS NEHRP contract from the external grants program to install a borehole seismometer near the Factor building. We met with Facilities and have an agreement with Botany to install a 50-100 m borehole sensor in the Botanical Gardens parking lot next to Factor. A priority next year is to obtain a drilling contract, and to get the drilling and seismometer installation under way.

ACCOMPLISHMENTS

The Kinemetrics/Quanterra equipment was delivered and the UCLA Factor building was rewired to interface with the new equipment (Fig. 1).

Figure 1. Quanterra 24 bit 500 sps installed in Factor

We have achieved out goal of recording 72 channels of seismic data real-time from the building which can be viewed on the Web (presently but not permanently) at http://teren.ess.ucla.edu:5813 password: seismo. The window displays the 72 channels in real-time (Fig. 2) with basement channels at the bottom to roof channels at the top.

Figure 2. (a) Real time site (b) Seismometers in Factor Building (c) USGS site

When an event occurs the seismic waves can be viewed as they pass up the building. The location and timing of the event can be obtained from the USGS real-time location web site http://quake.wr.usgs.gov/recenteqs/latest.htm (Fig 2).

Installing new equipment in the Factor building (Fig. 2) involved mounting nine Quanterra 4128 digitizers, constructing enclosures, wiring of all 72 data channels, configuration of the data acquisition software, and building a 1.5 TB array for local data storage to prevent a seismic data loss in the case of network outages (earthquakes, network equipment malfunctioning, etc.). The wiring was finished the day before Christmas (2003) and since then the system has functioned flawlessly. Currently, the system is configured to send 500 sps data to the Factor RAID and 100 sps to the department RAID. The system generates 3 Gbytes/day. The Antelope real-time seismic software has paid dividends in getting this system going smoothly.

Data from the network has been used to identify building softening effects associated with high amplitude shaking (Kohler et al., 2004) from two earthquakes, the 9/3/02 Yorba Linda and 3/28/03 Encino events. The first mode frequencies measured from earthquake data are about 0.05 to 0.1 Hz lower than those corresponding to ambient vibration recordings indicating softening of the building as amplitudes become larger. The frequencies revert back to pre-earthquake levels within 15 minutes of the Yorba Linda earthquake. Shaking due to strong winds that occurred during the Encino earthquake dominates the frequency decrease which correlates in time with the duration of the strong winds. The first shear wave recorded from the Encino and Yorba Linda earthquakes takes about 0.4 sec to travel up the 17-story building. Future work will involve assessing how much of the non-linearity is structural versus how much is due to non-linear effects in the soil.

Partnerships: We developed a partnership with USGS Structural Engineer, Dr. Erdal Safak, and wrote a joint publication. We also developed contact with Prof. Mark Hansen, UCLA Statistics Department, and his graduate student Andrew Baek who are interested in analyzing Factor data. We have set up a collaboration with Mladen Vucetic, UCLA Civil Engineering, who has agreed to analyze soil samples from the borehole for nonlinear changes in modulus and attenuation as a function of strain amplitude. We are also interacting with Jamie Steidl, UCSB, who is advising us in the borehole installation.

FUTURE GOALS AND OBJECTIVES

The UCLA seismic array can serve as an excellent outreach tool but further programming of the web site is required. We need to label and calibrate some channels, and to enable backward scrolling in order to view records from the last quake. We need to develop an automatic alerting system when an earthquake occurs. Erdal Safak (USGS engineer) is prepared to assist in implementing his building animation software. The following organizations would probably be inclined to have links from their web sites to the Factor site as well as CENS: USGS-ANSS, SCEC, and IRIS.

Much of the added work we propose for the borehole and free-field stations will be funded by the US Geological Survey. However this array extends the CENS network. Data mining as well as comparison with the USC wireless structural monitoring array (S5) will be performed under CENS.

Timeline

Install bore hole seismometer: June, 2004.
Install free-field stations (4 in array and one at borehole wellhead): February, 2005.
Analyze data from Factor Network (earthquake-dependent): present - August, 2005.
Characterize foundation soil in terms of non-linear effects: present - December, 2004.

PEOPLE

FACULTY

Paul Davis

STAFF

M. Kohler, Prof. Researcher
Igor Stubailo, Engineer

GRADUATE STUDENTS

Allen Husker

ASSOCIATES

Erdal Safak, USGS